The oxydihydrotrizaine antimalarial WR-99210 is active against strains of Plasmodium berghei resistent to cycloguanil and pyrimethamine. The presence of a masked hydroxylamine function and a 2,4,5-trichlorophenoxy residue in this interesting compound are unappealing and suggested a chemical investigation. Treatment of WR-99210 with refluxing 1N HC1 fragments the molecule into an acetone oxime ether derivative and a C2N4-moiety not yet identified. Further hydrolysis of the acetone oxime ether unmasks the hydroxylamine unit and afforded the corresponding hydroxylamine ether derivative. Cyclization of the former with cyano-guanidine gave the biguanide and its cyclization with acetone gave chemically pure WR-99210. The antimalarial activity of chemically pure WR-99210 and its biguanide precursor will be measured.